In recent years, as a technique relating to display apparatuses, a so-called monolithic liquid crystal display (hereinafter also referred to as a “system liquid crystal display”) has attracted attention in which peripheral device units, such as a drive circuit unit, a control circuit unit, and the like, are integrated with a pixel unit. In a semiconductor device used in such a system liquid crystal display, a switching element for the pixel unit, and the peripheral device unit are simultaneously formed on the same substrate, whereby the number of parts can be significantly reduced. Moreover, steps of assembling and inspecting the liquid crystal display can be reduced. Therefore, the manufacturing cost can be reduced, and the reliability can be improved.
For display apparatuses such as liquid crystal display apparatuses and the like, there is a strong demand for higher performance, such as lower power consumption, and higher definition and a faster response time of image display. There is also a demand for smaller space occupied by the peripheral device unit, and integration of higher-level systems (a memory, a signal processing circuit, etc.).
Therefore, for semiconductor devices used in display apparatuses, there is a strong demand for a still smaller size of each element, and the peripheral device unit requires submicrometer design rules, i.e., fine pattern accuracy at the integrated circuit (hereinafter also referred to as “IC”) level so that a larger number of elements are formed in a limited area. For semiconductor elements constituting the peripheral device unit, there is also a demand for a higher mobility of carriers in a semiconductor layer. To meet this demand, the reduction in size of each element is also required.
However, in conventional manufacturing processes which form a semiconductor device directly on a glass substrate, the insufficient heat resistance of the glass substrate may lead to a distortion in the glass substrate in a thermal treatment of the manufacturing process, and therefore, a desired submicrometer circuit pattern may not be formed. Glass substrates used in manufacture of liquid crystal display apparatuses, such as system liquid crystal displays and the like, are becoming larger, resulting in an in-plane distortion in the glass substrate being more likely to occur during the manufacturing process.
In contrast to this, there is a technique of using a silicon-on-insulator (SOI) substrate in which an integrated circuit is formed in a monocrystalline silicon layer provided on an electrical insulator, and transferring a peripheral device unit onto a substrate of a liquid crystal display. With this method, conventional IC chip fabricating processes can be used to form an integrated circuit including semiconductor elements. Therefore, a semiconductor device including a minute and high-performance integrated circuit which has a desired submicrometer circuit design can be achieved. However, when the peripheral device unit is transferred onto the substrate, then if a surface of the substrate on which the transfer is performed is not flat, it is difficult to reliably attach the peripheral device unit to the substrate.
To address such a problem, as described in, for example, Patent Document 1, a technique has been studied and developed for forming gate electrodes of the pixel unit, removing an insulating film and a sacrificial film, by etching, from a region in which a peripheral device unit is to be provided, and attaching a semiconductor device having semiconductor elements to the region. With this technique, the microroughness of the glass surface is reduced by performing etching using an etchant which provides a selectivity ratio with respect to the glass substrate when the sacrificial film is removed. Moreover, it is considered that the bonding strength after the transfer is satisfactorily high compared to a process in which the sacrificial film is not formed (only the insulating film is removed by etching to expose the glass surface).